Method and apparatus for forming and drawing fused metal-oxide tubes

ABSTRACT

A technique and apparatus for forming a fused metal-oxide structure, for example a fused silica tubing, by depositing a metal oxide on a deposition mandrel by vapor phase hydrolysis, heating the thus-deposited metal to a temperature sufficiently high to form a viscous glass melt, and drawing the structure from this glass melt.

Unite States Pete Foster Lee Gray D Tex.

Dec. 18, 1969 Nov. 16, 1971 Texas merit lInr-ated Dallas, Tex.

lnventor Appl. No. Filed Patented Assignee METHOD AND APPARATUS FOR FOGAND DRAWING FUSED METAL-OXIDE TUBES 18 Claims, 9 Drawing Figs.

US. Cl "C 65/86,

Int. Cl. C031) 17/0 3 Field of Search 65/86, 87, 88, 89, 66,141, 375,DIG. 8; 23/273 SP, 301 SP [56] Refierenoes Cited UNITED STATES PATENTS2,188,121 1/1940 Smith 65/86X 3,268,321 8/1966 Chapman 65/66 X 3,401,0289/1968 Morrill, Jr. 65/88 X Primary Examiner-Arthur D. KelloggAttorneys-Samuel M. Mims, .lr., James 0. Dixon, Andrew M. l-lassell,Harold Levine, Melvin Sharp, William E. Hiller and John E. VandigriffABSTRACT: A technique and apparatus for forming a fused metal-oxidestructure, for example a fused silica tubing, by depositing a metaloxide on a deposition mandrel by vapor phase hydrolysis, heating thethus-deposited metal to a temperature sufficiently high to form aviscous glass melt, and drawing the structure from this glass melt.

PATENTEU 16 3,820,704

SHEET 2 BF 3 INVENTOH FOSTER L. GRAY PATENTEUNUV 1s \97l SHEET 3 (IF 3INVENTOR. FOSTER L. GRAY METHOD AND APPARATUS FOR FORMING AND DRAWINGFUSED METAL-m]? TUBES This invention relates to the formation of metaloxide structures and specifically to a method of drawing fusedmetaloxide structures from a vapor phase hydrolysis deposition surface.

Various methods are known in the prior art for the manufacture of fusedmetal-oxide structures by drawing structure, for example, a sheet ortubing, from a viscous liquid melt of the metal oxide. Among theearliest example of a workable apparatus for producing a tubular silicastructure is disclosed in U.S. Pat. No. 2,l55,l3l. In that patent,quartz is fed into a melting chamber. A die having an annular shape ispositioned at the bottom of the melt chamber and a glass tube iswithdrawn through the die. An inert gas is fed into the center of theannular chamber to aid in prevention of the formation of crystallinequartz on the interior of the tube wall. The melting chamber is made ofa material having a high melting point which is subject to littlechemical attack by the molten quartz, for example, tantalum, tungsten ormolybdenum. The disadvantages of method and apparatus reside in the highcost of the melt chamber material and in the fact that a preliminaryoperation of quartz production is necessary. U.S. Pat. No. 2,398,952discloses an apparatus for the production of silica glass sheets. Bythis method powdered silica is fed to a melting chamber where a seriesof heating elements melts the powder. The glass melt is fed through anelongated aperture from which a glass sheet is drawn. An inherentdisadvantage in this method of producing glass sheets is that gases areentrapped in the glass melt, appearing as bubbles in the final product.Additionally, a separate process for making the powdered silica isnecessary. A third apparatus and method for producing glass tubing isdescribed and illustrated in U.S. Pat. No. 2,979,864. In the latterpatent, a glass melt is fed in the form of a stream onto a conicallyshaped mandrel. The chamber surrounding the mandrel is heated. A tube isthen drawn from the small end of the material. ln U.S. Pat. No.3,261,676 another method for forming silica tubing is disclosed. In thismethod, granulated or powdered silica is fed to a heated chamber ismelted, and is drawn through an annular die. This method again promotesthe formation of bubbles in the final fused silica article and alsorequires that the powdered silica be produced in a separate operation.

It is therefore desirable to possess an apparatus and method for theformation of metal-oxide structures, for example fused silicon dioxidestructures, which eliminates the disadvantages of the prior artprocesses and apparatus. A desirable method for the production ofmetal-oxide articles will simultaneously and continuously produce thedesired metal oxide and form a fused structure therefrom. In addition,the method and apparatus will produce a structure which has a veryhigh-purity metal-oxide content. The structure will be clear or milky asdesired, but will contain little or no entrapped gases or otherimpurities.

To achieve the foregoing attributes, this invention therefore provides amethod for continuously forming a structure from a fused oxide of ametal comprising depositing a metal oxide in a deposition zone upon adeposition surface, heating the metal oxide to a temperature which willfuse the oxide to form a plastic, vitreous mass, drawing a continuousstructure from the plastic, vitreous, mass cooling the mass to atemperature below which it becomes rigid. In a preferred embodiment ofthe invention a tubular silicon dioxide structure is drawn from amandrel of circular cross section.

In addition, a machine for performing this method is also providedincluding an apparatus for continuously drawing a structure from a fusedplastic mass of metal oxide comprising a support member. a mandrelrotatable associated with the support member, means for rotating themandrel, means for depositing a fused, plastic metal oxide mass on adeposition surface of the mandrel by vapor phase hydrolysis of a metalhalide, means for continuously drawing a fused metal-oxide structurefrom the plastic mass.

The foregoing method and apparatus of this invention will be morereadily understood by reference to the attached drawings in which:

FIG. 1 is an apparatus for carrying out a preferred embodiment of theinvention by drawing a tubular structure from a plastic, silicon dioxidemelt;

FIG. 2 is a partially schematic cross-sectional view of a vapor phasehydrolysis torch for use with the apparatus of FIG. 1;

FIG. 3 is a front view of the torch of FIG. 2;

FIG. 4 is a representation of the manner in which a mandrel is baitedprior to the tube-drawing process;

FIG. 5 is an illustration of another method of baiting the mandrel priorto the tubedrawing process;

FIG. 6 is a representation of another embodiment of the invention fordrawing a tubular structure from a mandrel;

FIG. 7 is yet another embodiment of this invention for drawing a tubularstructure from a cylindrical mandrel;

FIG. 8 illustrates a secondary step of drawing the tubular structurethrough a die before the structure is cooled below its plastictemperature FIG. 9 is a top view of the die shown in FIG. 6 with thedrawn tube omitted.

The invention will be described in relation to a preferred embodiment ofproducing fused silicon dioxide tubing. It is to be understood thatvarious other hollow shapes can be made by the method of this invention.For example, as shown in FIGS. 6 and 7, a square tube can be produced byutilizing the method disclosed herein and subsequently drawing thetubing through a die of desired shape. In addition, a variety of othermetal-oxide structures can be made by the method of this invention. Themetal oxides must, however, be ductile at elevated temperatures, i.e.,it must be possible to form a melt around a mandrel and draw a hollowstructure therefrom. Metal oxides which transcend directly from a solidto a nonviscous liquid are not within the scope of this invention.Various metal oxides, the within of which are selected from groups "A,"LA, IVA, llIB, W8 and VB of the Periodic Table (as it appears on theflyleaf of Perry'Chemical Engineers Handbook, edited by R. H. Perry, C.H. Chilton, and S. D. Kirkpatrick, 1963, McGraw-Hill Book Company, lnc.,New York) and which meet the above requirement of ductility are useablewithin the scope of this invention.

Referring now to FIG. 1, an apparatus for producing fused silicondioxide tubing is illustrated. A mandrel i0 is rotatably mounted tobracket 12 which is attached to hydrolysis enclosure 14. A suitablebushing 16 connects bracket 12 and mandrel l0. Enclosure 14 is made of asuitable refractory mate rial and can be lined with graphite. Mandrel 10is substantially of cylindrical configuration and has a bottom portion18 of frustoconical. In this embodiment, the mandrel l0 and its bottomportion l8 are made of graphite. Other mandrel compositions can be used,as will be described hereinafter. Vapor phase hydrolysis torches,generally designated as 20, are mounted on bracket members 22 andpositioned to direct their flame 24 toward the frustoconical portion 18of the mandrel 10. A resistance heating element 26 surrounds the mandreland extends above and below the frustoconical portion 18. The mandrel isrotated by a motor 28 through belt and pulley arrangement 30.

The vapor phase hydrolysis torches 20 are connected to suitable sourcesof combustible gas and to a suitable source of a metal halide. A metalhalide is hydrolyzed in flame 24 to a metal oxide. The metal oxide isdeposited on mandrel 10 as a glassy or vitreous plastic melt 32. Asuitable hydrolysis torch is illustrated and described in conjunctionwith FIGS. 2 and 3 hereafter. The resistance heating coil 26 isconnected through electrodes 34 to a suitable controlled electricalenergy source. The temperature of the glass melt 32 must be kept at sucha eve-i to maintain the melt in a plastic state so that it can be drawninto a desired shape. For example, the temperature of a silicon dioxidemelt is maintained in the range of from l,800 to 2,200 and preferablynear 2,000" C. The temperature of the area inside resistance coil 26 ismaintained by that coil at around L500 C. The heat from the vapor phasehydrolysis torch flame 24 elevates the deposition zone and consequentlythe glass melt 32 to a a temperature within the required range of 1,800to 2,200 C. If desired, the electrical resistance heating element 26 canbe eliminated, since the torch flame 24 can be adjusted to maintain theglass melt within the required temperature range. However, more uniformheat control can be achieved when an auxiliary resistance heater such asthat shown as 26 is utilized.

In the preferred embodiment of this invention, a silicon dioxide tube isproduced. A tube bait 36 surround the upper portion of the mandrel I0.It rotates with the mandrel, but remains longitudinally stationary. Asthe mandrel 10 rotates, silicon dioxide being deposited by the vaporphase hydrolysis flame 24 is evenly distributed around the circumferenceof the mandrel l and the lower frustoconical die or end portion 18 ofthe mandrel. The lower portion of the mandrel is so shaped to preventthe glass melt 32 from sliding off the deposition zone. The shape of thelower portion of the mandrel depends, upon, among other things, thedegree of adhesion of the various metal oxides to the mandrel and themandrel composition itself. A tube 38 is drawn from the plastic,vitreous melt 32 contained on the lower portion 18 of the mandrel. Thetube 38 will neck slightly in an area 40 immediately below the mandrel.It will assume a solid tubular configuration at a predetermined distancebelow the mandrel. This distance is dependent on the variety of factorsincluding draw rate, temperature of the glass melt, the metal-oxidecomposition and the temperature of the inert gas inside the tube.

The enclosure I4 is mounted by members 42 on a pulling assembly 48. Thetube 38 extends through an opening 44 in enclosure 14 and is infrictional contact with pulling rolls 46, which are mounted for rotationin the direction of the arrows on pulling assembly 48. The pullingassembly is mounted on suitable legs 50 extending down to a suitablefloor or base. After tube 38 passes through pulling rolls 46, it is fedinto a tank 52 containing a liquid 54, for example, water. The water orother suitable liquid serves a dual purpose. First, it quenches the tube38 to a temperature low enough to ensure safe handling. Secondly, itprovides a convenient seal for the hollow interior of tube 38. Such aseal is required because of the necessity of introducing nonoxidizinggas into the hollow interior of the tubular structure 38. To preventoxidation of the mandrel surface, especially if the mandrel is composedof graphite, and to prevent impurity formation on the interior surfaceof the tube, a suitable inert or nonoxidizing gas, for example,nitrogen, is introduced into conduit 56 through a rotating seal member58 and into channel 60 which runs through the center of mandrel 10.Channel 60 communicates with the interior of the tubular structure 38.The pressure of the gas inside the tubular structure 38 is maintained atslightly above atmospheric, for example, at a gauge pressure of 1 inchof water. The gas travels up interface 62 between the glass melt and themandrel, thus forming a protective layer to prevent oxidation of thegraphite and impurity formation of the tube. The gas is allowed toescape as necessary through the space between tube bait 36 and mandrel10.

The lower portion of the tubing 66 can be conveniently scored byconventional techniques afier it passes through pulling rolls 46. Whenthe score reaches a level below the surface of the fluid 54, the lowerportion of the tube can be broken away and removed from the liquid.Normally an apparatus such as that illustrated in FIG. I can have areceiving tank 52 which is over 40 feet long, thus allowing thecontinuous production of 40-foot lengths of tubing. It can readily beseen, through, that longer or shorter lengths can be produced asdescribed. Referring now to FIGS. 2 and 3, a suitable vapor phasehydrolysis torch for practice of this invention is illustrated. It willbe described with reference to hydrolysis of silicon tetrachloride tosilicon dioxide by the following reaction.

It is to be understood, however, that various liquid metal halides,preferably chlorides, can be utilized with this torch for hydrolysis tothe corresponding metal oxide. This torch is similar to that disclosedin a copending application to Herbert J. Moltzan, Ser. No. 744,153 filedJuly ll, 1968, now pat. No. 3,565,345

Referring to FIGS. 2 and 3, tube or pipe 80, preferably constructed fromstainless steel, extends through the length of the torch, generallydesignated as 20, to provide a passage for vaporized silicontetrachloride entrained in a carrier gas. A T- connection designatedgenerally by the numeral 82 is connected about the tube 80 and is sealedat one end to the tube 80 by collar member 84. A coupling member 86 fitsover a stainless steel tube 88 to provide an annular sheath chamber 90between the tube 80 and the tube 88. An inlet portion 9| of T-connection82 is connected to a source of sheath gas in a manner to be laterdescribed, which here can be oxygen-containing gas. This sheath gas ispassed into the annular sheath chamber 90.

A mixing chamber 92 is formed by chamber walls 94. An inlet fitting 96is adapted to be connected to a source of a first combustible gas, whileinlet 98 is adapted to be connected to a source of a second gas. Thecombustible gases are mixed within chamber 92 in order to limit anypossible flashback to the torch housing. An outer annular chamber 100 isformed by annular walls 102 to define the cooling chamber about thetorch. An inlet fitting 104 is connected to a suitable supply of cooledfluid, for example water, which is circulated through the chamber 100and exhausted via outlet fitting 106.

Oxygen is supplied through a conduit 108 to the inlet of threeflowmeters 112, [I4 and 116. Hydrogen is supplied via a conduit II8 to aflow meter 120. Both the oxygen and hydrogen are dried prior to enteringthe flowmeters. Suitable valves are provided at the output of each ofthe flowmeters in order to allow accurate regulation of the flow rate ofthe gases to the torch. Oxygen is supplied from flowmeter 112 through aconduit 122 to the inlet portion 91 of the T-connection member 82.Oxygen is supplied from flowmeter 116 through a conduit 146 to the inlet96 of mixing chamber 92. Hydrogen is supplied from flowmeter 120 throughconduit 148 to the inlet 98 mixing chamber 92.

Oxygen from flowmeter 114 is supplied through a conduit 124 to a bubblerunit 126. The bubbler unit contains liquid silicon tetrachloride. Theoxygen from conduit I24 passes through difiuser member 127, and passesup through the liquid silicon tetrachloride. As it does, liquid silicontetrachloride is entrained or dissolved in the oxygen and is carried outthrough conduit 128 into the inlet I30 of tube 80. It is to beunderstood that a conventional vaporizer or diffusertype unit can beused to replace the bubbler unit 126.

A nozzle assembly I50 is attached to the face of the torch 20 by screwsI52. As shown in FIGS 2 and 3, four screws I52 pass through the nozzleassembly I50 and into portions of the walls defining chamber I00. NozzleI50 comprises a unitary circular member having a center opening I54 forreceiving the end of pipes 80 and 88. As best shown in FIG. 2, the endof the pipe 80 is closed with the exception of a center nozzle aperture156 defined therein. Due to the difference in the diameters of pipe 80and pipe 88, an annular opening I58 is defined concentrically about thenozzle aperture 156. A plurality of nozzle openings 160 are definedthrough the nozzle assembly 150. The diameter of these openings isgenerally the same or smaller than the diameter of the nozzle apertureI56.

In operation of the torch 20, silicon tetrachloride entrained in oxygencarrier gas is passed through the pipe 80 and out of the aperture 156 asa gaseous jet stream. A concentric sheath of oxygen is passed throughannular opening 158. Eight streams of combustible mixture of hydrogenand oxygen are directed at an angle toward the axis of the jet streamfor penetration of the gas sheath and interaction with the gaseous metalchloride. When the torch is ignited, combustion occurs at this regionand the silicon tetrachloride is decomposed by vapor phase hydrolysis tofonn silicon dioxide. The flame is directed toward the glass melt 32 onthe deposition mandrel (FIG. 1) and becomes part of the plastic,vitreous silicon dioxide mass surrounding the frustoconical portion 18of the mandrel.

FIGS. 4 and 5 illustrate themanner in which a mandrel is baited, i.e.,how the tube drawing process is initiated. FIG. 4

I illustrates the-procedure for baiting a graphite mandrel or themandrel 163 is heated sufficiently to become plastic, rotation of thetube bait 164 is begun. As a glass melt (indicated as 32 in FIG. 1) isbuilt up, pull rolls 165 begin to draw a newly formed tube from themandrel base. Usually, the tubular metal oxide structure will begin toneck, i.e., reduce in diameter, immediately below the mandrel 163.Hence, it is necessary to reduce the distance between the pull rolls 165to conform to the tube diameter. Conventional adjustment mechanisms areutilized to accomplish this and to maintain a constant tension or pullon the newly formed tube. q

FIG. 5 illustrates another method for baiting a mandrel coated with, forexample, silicon carbide. Such a coating will not react with silicondioxide or other metal oxide being deposited to form impurities on theinterior of the tubular structure. Since it is not necessary to protectthe upper portion of the mandrel 166, a tube bait 167 is formed aboutthe mandrel as illustrated. The bottom of the tube bait can be sealed asshown at 168 or can be extended down'into a liquid tank as shown inFIG. 1. Conventional pull roll adjustment is utilized as required.

FIG. 6 illustrates the deposition procedure utilizing a mandrel 169 ofgraphite'coated with a nonreactive substance, against for example,silicon carbide. The baiting procedure described in conjunction withFIG. 5 is utilized in this embodiment. The fused metal-oxide melt 170 isdepositedaround the frustoconical portion of the mandrel I69 and thetube is drawn therefrom by pull rolls 171.

FIG. 7 illustrates yet another embodiment utilizing a mandrel which willbe wetted by the fused metal-oxide melt. This type of mandrel designates172 composed of, for example, platinum is substantially cylindricalthroughout its entire length. As set forth above, the use of afrustoconical end portion is dependent upon the temperature at which theglass melt is maintained, the degree of wetting of the mandrel surfaceby the metal oxide melt, the pressure of the nonoxidizing gas within thetubular structure, and the rate at which the tube is withdrawn from themandrel by pull rolls 173. These factors also control the diameter andwall thickness of the tubular structure produced by the method of thisinvention. A metaloxide tube 174 can be drawn from mandrel 172 withlittle or not necking below the bottom of the mandrel.

Referring now to FIG. 8, a secondary operation on the newly formed tubeis illustrated. It is performed immediately after the tubular structureis drawn from the mandrel. In this embodiment, vapor phase hydrolysistorches 180 deposit a plastic mass of fused metal oxide 182 on mandrel184. The mandrel 184 is rotatably mounted through the top 186 of thedeposition furnace. A seal 188 is'interposed between the mandrel 184 andthe top 186 to prevent leakage from the interior of the depositionfurnace. A suitable heat shield 190 is mounted around the depositionzone and rests on the furnace floors 192. Apertures 194 are provided inthe heat shield 190 to allow the torch flame 196 to reach the depositionarea where the fused metal oxide 182 is deposited. The sides of thefurnace comprise a quartz tube envelope 198 in which the torches 180 aresealingly mounted. Surrounding the quartz envelope is an inductingheating source illustrated as coils 200.

Coils 200 are connected to, for example, a radiofrequency energy source.In this embodiment, the mandrel itself is.

heated to a' temperature sutficient when combined with the heat of thehydrolysis flame 196 to maintain the metal-oxide melt 182 in a plasticstate. A die 202 mounted in mating grooves in the floor 192 of thefurnace has an orifice 204 which is positioned immediately below themandrel 184. The die can be heated by induction, can be unheated, or canbe cooled as desired, depending upon the particular metal-oxidestructure being formed and the properties desired therein. A typicaldieconfiguration is shown in FIG. 9 as having a rectangular opening ororifice 204.

In operation of the embodiment shown in FIG. 8, the fused metal-oxidemass 182 is first drawn into'a somewhat tubular configuration. While themetal oxide is still in a plastic state, it-

is pulled through die orifice 204 where it conforms to the shape of theparticular orifice being utilized, here to a rectangular shape. In thisembodiment, the mandrel 184 would be baited as shown in FIG. 5, exceptthat the, bait would be positioned through the die and would besufiiciently small to ensure initial passage through the die. Anonoxidizing gas is also fed through channel as described in conjunctionwith FIG. l

To ensure a better understanding of the invention, an exemplificationfollows wherein a graphite mandrel is vertically positioned in adeposition fumace lined with a suitable refractory material. The mandrelhas a diameter through the main body portion of 3 inches. The mandrelhas a bottom portion shaped substantially like a frustum of a cone, thedepth of the frustum being 3% inches, the major diameter being 3% inchesand the minor diameter being the same as that of the main body of thetube? The tube has a bore through its axis and is connected to anitrogen supply outside the deposition furnace. The mandrel is rotatedabout its axis at about 20 r.p.m. Two vapor phase hydrolysis torches arepositioned in a radial direction from the mandrel so that the nozzles ofthe torches are about 3% inches front the mandrel surface. The'torchesare positioned such that the metal oxide produced by vapor phasehydrolysis is deposited on the frustoconical section of the mandrel in azone contiguous to its attachment point to the main body portion of themandrel.

A fused silica tube bait is positioned about the mandrel'and into a setof pulling rolls in a manner similar to that shown in FIG. 4. The bottomportion of the fused silica tube bail is sealed. Elementaihydrogen andoxygen are used as the the combustible gaseous mixture. Silicontetrachloride in oxygen carrier gas is also supplied to the torch. Thefurnace isfmaintained at l,550 C. by resistive heating elementspositioned around the mandrel. The deposition zone is maintained at atemperature of abou2,000 0C., augmented by the flames of the vapor phasehydrolysis torch. Elemental nitrogen is introduced through'the mandrelbore into the interior. of the tube bait. The nitrogen effectivelyprevents reaction between the graphite mandrel and the glass melt in thedeposition zone. The pressure of the nitrogen is maintained slightlyabove atmospheric throughout the entire procedure. After the torches areignited, vaporous silicon tetrachloride is fed into the flame of eachtorch an: flow rate of 40 grams of silicon tetrachloride per minute,depositing silicon dioxide, at a rate of about 7.1 grams perminute pertorch. A toroidally shaped glass melt is formed around the intersectionof the cylindrical and conical sections of the mandrel. When thethickness of the melt reaches about one-quarter to one-half inch, thepulling rolls are started and the fused silica tube is withdrawn fromthe mandrel at a rate consistent with the deposition rate.

A tube of uniform outside diameter and wall thickness of about 2.5 mm.is produced. The 0D tolerances are plus or minus about 0.5 mm. The tubeis transparent and compares favorably in quality with those produced byconventional methods.

Although the foregoing invention and example thereof have been describedin relation to a specific embodiment, it is readily apparent, to one ofordinary skill that many variations of the present invention can be madewithout departing from its spirit or intent. The invention is to limitedonly as defined in the appended claims.

What is claimed is:

1. An apparatus for continuously drawing a hollow tubelike structurefrom a fused plastic mass of metal oxide, comprising:

a. a support member;

b. A vertically suspended substantially cylindrical mandrel rotatablyassociated with the support member near the upper end of said mandrel;

c. means for rotating said mandrel;

d. means for depositing a fused, plastic metal-oxide mass on adeposition surface of the mandrel near the lower end thereof by vaporphase hydrolysis of a metal halide; and heating means for maintainingsaid deposited oxide in a plastic state.

e. means for continuously drawing a hollow tubelike fused metal-oxidestructure downward from the fused oxide mass surrounding the lower endof said mandrel.

2. The apparatus of claim 1 wherein said mandrel is composed ofplatinum.

3. The apparatus of claim 1 wherein said mandrel has a frustoconical dieportion near the lower end thereof, the major circumference of the dieportion being the lower end of the mandrel over which the structure isdrawn.

4. The apparatus of claim 1 including means for circulating anonoxidizing gas between the interior surface of said structure and thesurface of the mandrel.

5. The apparatus of claim 3 wherein said mandrel, including the die.portion thereof, is composed of graphite coated with silicon carbide.

6. The apparatus of claim 1 wherein the means for depositing comprises aplurality of vapor phase hydrolysis torches attached to the supportmember and positioned to deposit a metal oxide on the mandrel, saidtorches including means for feeding vaporous metal halide andcombustible gas to each thereof.

7. The apparatus of claim 6 wherein the said torches each include acentrally located aperture for directing a gaseous stream vof metalhalide, circumvented by an aperture for directing a gaseous stream ofoxygen and further circumvented by a plurality of apertures forselectively directing gaseous streams of a combustible gas.

8. The apparatus of claim 1 further including auxiliary heating meanscircumventing said mandrel.

9. An apparatus for continuously drawing a hollow tubelike structurefrom a fused plastic mass of metal oxide, comprising:

a. a support member;

b. a vertically suspended substantially cylindrical mandrel rotatablyassociated with the support member near the upper end of said mandrel,said mandrel having a frustoconical die portion near the lower endthereof with a major circumference at the lower end thereof;

c. means for rotating said mandrel; d. a plurality of vapor phasehydrolysis torches attached to said support member and positioned todeposit a vitreous metal oxide on the mandrel, each of said torchesdirecting a flaming gaseous stream radially inward toward said mandrel,said gaseous stream comprising oxygen, a combustible gas and a vaporousmetal halide;

e. heating means for maintaining said deposited oxide in a plasticstate; and

f. means for continuously drawing a hollow tubelike fused metal-oxidestructure downward from the major circumference of said die portion.

10. A method for continuously forming a hollow tubelike structure from afused metal oxide, comprising:

a. depositing a plastic, vitreous mass of a metal oxide by vapor phasehydrolysis on a deposition zone of a selectively shaped verticallysuspended mandrel;

b. rotating said mandrel about its vertical axis;

c. continuously drawing a hollow tubelike structure downward from theplastic oxide mass surrounding the lower end of said mandrel as themetal oxide is deposited;

and

d. cooling the drawn structure to a temperature below which it becomesrigid.

11. The method of claim 10 wherein said metal oxide is deposited from aflaming gaseous stream comprising oxygen, a combustible gas and agaseous metallic compound.

12. The method of claim 11 wherein said metallic compound is silicontetrachloride.

13. The method of claim 10 wherein said mandrel has afrustoconical-shaped lower end portion with a major circumference at thelower end thereof.

14. The method of claim 10 further comprising heating said depositionzone with an auxiliary heat source circumventing said deposition zone.

15. The method of claim 10, wherein the deposition zone is heated to atemperature within the range of from 1,800 to 2,200 C.

16. The method of claim 14 wherein said auxiliary heat source is aradiofrequency induction source and said mandrel serves as a susceptor.

17. The method of claim 10 wherein the drawn end of the tube likestructure is sealed and the method further includes introducing anonoxidizing gas into the interior of the drawn structure.

18. The method of claim 10 further including the step of drawing thestructure through a selectively shaped die while the structure is stillin a plastic state.

I! i l i

2. The apparatus of claim 1 wherein said mandrel is composed ofplatinum.
 3. The apparatus of claim 1 wherein said mandrel has afrustoconicAl die portion near the lower end thereof, the majorcircumference of the die portion being the lower end of the mandrel overwhich the structure is drawn.
 4. The apparatus of claim 1 includingmeans for circulating a nonoxidizing gas between the interior surface ofsaid structure and the surface of the mandrel.
 5. The apparatus of claim3 wherein said mandrel, including the die portion thereof, is composedof graphite coated with silicon carbide.
 6. The apparatus of claim 1wherein the means for depositing comprises a plurality of vapor phasehydrolysis torches attached to the support member and positioned todeposit a metal oxide on the mandrel, said torches including means forfeeding vaporous metal halide and combustible gas to each thereof. 7.The apparatus of claim 6 wherein the said torches each include acentrally located aperture for directing a gaseous stream of metalhalide, circumvented by an aperture for directing a gaseous stream ofoxygen and further circumvented by a plurality of apertures forselectively directing gaseous streams of a combustible gas.
 8. Theapparatus of claim 1 further including auxiliary heating meanscircumventing said mandrel.
 9. An apparatus for continuously drawing ahollow tubelike structure from a fused plastic mass of metal oxide,comprising: a. a support member; b. a vertically suspended substantiallycylindrical mandrel rotatably associated with the support member nearthe upper end of said mandrel, said mandrel having a frustoconical dieportion near the lower end thereof with a major circumference at thelower end thereof; c. means for rotating said mandrel; d. a plurality ofvapor phase hydrolysis torches attached to said support member andpositioned to deposit a vitreous metal oxide on the mandrel, each ofsaid torches directing a flaming gaseous stream radially inward towardsaid mandrel, said gaseous stream comprising oxygen, a combustible gasand a vaporous metal halide; e. heating means for maintaining saiddeposited oxide in a plastic state; and f. means for continuouslydrawing a hollow tubelike fused metal-oxide structure downward from themajor circumference of said die portion.
 10. A method for continuouslyforming a hollow tubelike structure from a fused metal oxide,comprising: a. depositing a plastic, vitreous mass of a metal oxide byvapor phase hydrolysis on a deposition zone of a selectively shapedvertically suspended mandrel; b. rotating said mandrel about itsvertical axis; c. continuously drawing a hollow tubelike structuredownward from the plastic oxide mass surrounding the lower end of saidmandrel as the metal oxide is deposited; and d. cooling the drawnstructure to a temperature below which it becomes rigid.
 11. The methodof claim 10 wherein said metal oxide is deposited from a flaming gaseousstream comprising oxygen, a combustible gas and a gaseous metalliccompound.
 12. The method of claim 11 wherein said metallic compound issilicon tetrachloride.
 13. The method of claim 10 wherein said mandrelhas a frustoconical-shaped lower end portion with a major circumferenceat the lower end thereof.
 14. The method of claim 10 further comprisingheating said deposition zone with an auxiliary heat source circumventingsaid deposition zone.
 15. The method of claim 10, wherein the depositionzone is heated to a temperature within the range of from 1,800* to 2,200* C.
 16. The method of claim 14 wherein said auxiliary heat source isa radiofrequency induction source and said mandrel serves as asusceptor.
 17. The method of claim 10 wherein the drawn end of thetubelike structure is sealed and the method further includes introducinga nonoxidizing gas into the interior of the drawn structure.
 18. Themethod of claim 10 further including the step of drawing the structurethrough a selectively shaped die while the structure is still in aplastic state.